1. Field of the Invention
The present invention relates to an electrooptical display apparatus using an electrooptical element driven by an electrical signal and having memory characteristics and an electrooptical element driver used in the display apparatus and, more particularly, to a drive method for a non-display area outside an image forming area on the electrooptical element.
2. Related Background Art
Upon application of an electric field equal to or higher than a threshold value, an electrooptical medium having memory characteristics undergoes a desired switching operation, and thereafter, when the electric field is turned off or when an electric field below the threshold value is applied, the state after switching can be held. Since the medium having such characteristics can store information by the effect of its memory characteristics after the desired switching operation is performed by a write signal, it can be applied to, e.g., a large-capacity display element.
As a typical electrooptical medium having memory characteristics, a ferroelectric liquid crystal is known. The ferroelectric liquid crystal (FLC) is sealed between substrates subjected to a proper orientation treatment to prepare a cell having a liquid crystal layer which is thin enough to eliminate the spiral structure, thus providing two stable states having memory characteristics.
Such a liquid crystal cell can identify the two stable states to dark and bright states using at least one polarizer by utilizing birefringence of the liquid crystal. The switching operation between the two states is controlled by an electrical signal applied through electrodes formed by appropriately patterning the above-mentioned substrates.
In such a liquid crystal cell, in general, stripe-like scan electrodes are formed on one substrate, and stripe-like information electrodes are formed on the other electrode. Bright and dark states are written in pixels formed at crossing portions of the electrodes according to combinations of scan signals and information signals applied to these electrodes. In this manner, the liquid crystal cell is utilized as a display element.
When the electrooptical medium having memory characteristics such as an FLC is used as the display element, the following problems are posed.
More specifically, the display element is housed in a chassis or a casing to attain functions and safety, to protect an element electrical system, and to assure good outer appearance. In some cases, the display surface is concealed by the thickness of the chassis or the casing when it is viewed from an oblique direction. In order to avoid such cases, a non-display area is arranged around a display area, so that an effective display area can be prevented from being concealed unless it is watched from an angle outside a predetermined range.
However, with this arrangement, when such a non-display area is formed in a medium such as an FLC having memory characteristics, since the FLC is in an arbitrary state before an electrical signal exceeding a threshold value is applied to the FLC, the non-display area is not under control, and a display becomes nonuniform, thus considerably deteriorating the outer appearance in a practical use. Therefore, the non-display area must be controlled to a uniform state by an electrical signal. In this case, the memory characteristics need only satisfy image quality and a display function of the display element, and are not permanent. Therefore, a drive signal must be periodically applied.
Thus, the following technique is conventionally proposed. That is, electrodes for driving a non-display area are arranged around a display area, and an electrical signal is applied to the electrodes to drive a liquid crystal of the non-display area so as to realize a uniform display area (e.g., Japanese Laid-Open Patent Application No. 63-243994).
In the technique described in Japanese Laid-Open Patent Application No. 63-243994, however, as will be described later with reference to FIG. 3, since the non-display area is driven while interrupting a line scan operation of the display area after the line scan operation of the display area, the scan time is prolonged as compared to a case wherein only the display area is scanned, and the frame frequency of the display area is undesirably decreased.